Pour the heavy cream into a bowl and add the grain cereal bits. Submerge with the back of a spoon and allow to sit for at least an hour. Once the cream has absorbed enough flavor, sieve out the cereal and press the cereal bits to extract as much flavor as possible.
In a food processor or blender mix the marshmallow bits with the milk and allow to sit until they soften (this wont take more than 15 minutes or so). Once soft, blend the mixture.
In a large pot, mix the milk, and heavy cream with the sugar and salt. Once sugar has dissolved, add some of this mixture to the egg yolks to temper and whisk with the yolks. Repeat twice more. This is to temper the eggs.
Add the egg yolk mix to the pot and whisk to combine. Cook, stirring frequently, until the mix reaches 160 F, is slightly thickened.
Sieve the mixture into a heat-safe bowl to remove any egg or cereal bits. Add food coloring, if using, and stir to combine. Chill overnight.
Freeze, according to your ice cream maker’s instructions. In the last minute of freezing, add the additional marshmallow bits. Place in an ice cream tub (I use old pho containers) and store in your freezer.